When it comes to sex traits, brains are consistently inconsistent

Are differences between men and women reflected in their brains? For centuries, scientists would have said yes—and they’ve been searching for evidence of those differences since long before the invention of the MRI.

Now, the debate has taken an interesting twist: New research suggests that though there are brain differences between the sexes, there’s no such thing as a male or female brain, reports Stephanie Pappas for LiveScience.

The study is the first to look at sex differences in the whole brain, rather than just variations in different areas, writes Pappas. For a brain to be considered gendered, it must display multiple structures that can be identified as "male" or "female"—differences that in turn are consistently distinct between males and females themselves.

"Consider the peacock, with its sexually dimorphic tail," writes Pappas. "The difference in color and size is consistent between the sexes – there's no subset of peahens brandishing iridescent purple feathers."

To suss out whether such consistent differences exist, lead researcher Daphna Joel and her team examined over 1,400 MRI images of males and females. They identified 29 regions that often exhibit size differences and patterns of connectivity in individuals of different sexes, then looked for consistent differences in those brain regions across the sample MRIs.

That’s where the theory that brain differences are consistent across the sexes began to fall apart. Very few of the samples—six percent in a study of 281 brains from males and females and 2.4 percent in a study of over 600 brains—were internally consistent as “male” or “female.” Surprisingly, each brain studied had its own unique pattern of “maleness” and “femaleness.”

“Internal consistency is rare and is much less common than substantial variability,” the team writes. That turns stereotypes of brain differences on its head—and raises intriguing questions about the concept of gender. Psychologist John Barker tellsNew Scientist that “the study is very helpful in providing biological support for something that we’ve known for some time—that gender isn’t binary.”

But brain structure isn’t the whole story, writes Pappas: Rather, there’s mounting evidence that brain development in both genders responds to a mishmash of inputs like environment, genetics and external factors.

For now, though, brain structure doesn’t appear to be one of those factors—a fact that would have shocked scientists of yore. In 1882 Miss M.A. Hardaker wrote for Popular Science: "We have as much external evidence of the superiority of the masculine brain as of the superior breathing of the masculine lungs, or of the superior absorbing power of the masculine stomach." Though recent research proves these facts to be incorrect, the stereotypes entrenched in her work still loom large—in both society and in science.

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About Erin Blakemore

Erin Blakemore is a Boulder, Colorado-based journalist. Her work has appeared in publications like The Washington Post, TIME, mental_floss, Popular Science and JSTOR Daily. Learn more at erinblakemore.com.